The sensor head of this kind of crystal oscillator type of film thickness monitor is known, for example, in Patent Document 1. The sensor head in question is provided with a sensor head main body which has disposed therein a stepping motor (pulse motor for use in vacuum) as a driving means, and a holder which has disposed therein a plurality of crystal oscillators in the same circumference at a distance from one another, thereby being driven for rotation by the stepping motor. Provided that the direction from the sensor head main body toward the holder is defined as an upper side, the sensor head main body is provided with a mask body so as to cover that upper surface of the holder on which is disposed each of the crystal oscillators. In a predetermined position of the mask body, there is disposed a film-forming window to which faces one piece of the crystal oscillator.
In that portion of the sensor head main body which is positioned right under the film-forming window, there is disposed a first electrode. The first electrode is constituted by a plate-like member which is fixed, at one end thereof, to a support base provided in the sensor head main body. The holder is constituted by: a holding plate part made of stainless steel having formed, by recessing, on its upper surface a storing part to store therein each of the crystal oscillators; and a ring plate part, made of fluororesin, integrally fixed to a lower surface of the holding plate part. It is thus so arranged that vane type of electrodes which serve as second electrodes for electrical conduction with each of the crystal oscillators are held by the ring plate part. In this case, those parts of the vane type of electrodes which protrude downward from the ring plate part have dome-like contours. In this arrangement, when the holder is rotated by the stepping motor up to a phase (normal phase) in which one of the crystal oscillators and the film-forming window coincide with each other in the vertical direction, the second electrode comes into contact with a free end part of the first electrode, thereby bringing them into electrical conduction. By measuring a resonance frequency of the crystal oscillator in this state, the film thickness of a thin film to be formed on the crystal oscillator can be monitored. Then, for example, when the resonance frequency to be measured varies beyond a predetermined range, with an increase in the film thickness of the thin film to be formed, judgment is made that the crystal oscillator now in use has reached the end of its lifetime. Then, by means of the stepping motor, the holder is rotated once again to the phase in which the next crystal oscillator and the film-forming window coincide with each other in the vertical direction.
By the way, at the time of measuring the resonance frequency, the drive current to cause to flow through the crystal oscillator is generally as small as several mA. Therefore, taking into consideration the fact that the use is made in the vacuum atmosphere, as well as the resistance to corrosion and electrical resistance value, it is normal practice to use, as the first electrode and the second electrode, an article made of gold or an article in which gold-plated layer is formed on the surface of electrically conductive material may be used. However, if the portion of contact between the first electrode and the second electrode is both made of gold, i.e., the same kind of metal, the following findings have been obtained. Namely, the stepping motor is likely to get out of step and also, with an increase in the number of times of rotating the holder, the magnetizing current to flow through the stepping motor in order to drive to rotate the holder keeps on increasing. Then the following problem will occur in that the measurement of the film thickness of the thin film to be formed on the crystal oscillator cannot accurately be monitored: in case the stepping motor gets out of step to thereby give rise to a phase shift of the crystal oscillator relative to the film-forming window (i.e., as a result of deviation in position of stopping of the holder in the front-to-back direction of rotation, the crystal oscillator deviates in the front-to-back direction of rotation relative to the film-forming window) (as long as the resonance frequency does not deviate largely, the presence or absence of stepping out cannot be discriminated); and should the magnetizing current rise and the calorific value of the stepping motor increase, and this heat be received by the crystal oscillator.
Then, as a result of strenuous studies by the inventors of this application, the finding has finally been successfully obtained, especially, in that the above-mentioned phenomena are attributable to the fact that the surface layer portion (made of gold) of the first electrode gets roughened (becomes a surface with projections and recesses). In other words, in a phase in which any one of the crystal oscillators and the film-forming window coincide with each other in the vertical direction, the state of contact between the first electrode and the second electrode will be maintained by the urging force of the first electrode but, since the hardness of gold is relatively low, the contact portion between the first electrode and the second electrode will soon be in a state of getting adhered. If the holder is rotated in this state, the second electrode will slide along the surface of the first electrode in a manner to get peeled off by force. As a result of repetition of this sliding movement, the surface layer portion of the first electrode comes to get gradually roughened. As a consequence, it is considered that, by an increase in the sliding resistance (frictional resistance) of the second electrode relative to the first electrode during rotation of the holder, the stepping motor easily comes to become out of step, and further that the magnetizing current of the stepping motor keeps on increasing.